1. Field of the Invention
This invention relates to a pneumatic tire, and in particular to a pneumatic tire wherein the crown portion of the carcass thereof is reinforced by a circumferential belt layer comprising reinforcing elements arranged substantially in parallel with the equatorial plane of the tire in straight, wavy or zigzag form.
2. Description of the Prior Art
In pneumatic tires, many proposals have been made for strengthening the circumferential rigidity of the belt by the addition of a circumferential belt layer to an inclined belt layer comprising reinforcing elements inclined with respect to the equatorial plane of the tire.
For example, Japanese Patent Application Laid-Open No. 2-81706 discloses a tire comprising a circumferential belt layer for strengthening the circumferential rigidity which comprises wavy reinforcing elements and attaining vulcanization almost without preventing expansion of the diameter of the belt in tire manufacture. However, the addition of the circumferential belt layer causes an increase in the heat generation of the belt, because of not only increasing the thickness of the belt but also increasing the strain generated between the circumferential belt layer and inclined belt layer during rotation of the tire. Also the heat generation of the belt causes deterioration of the coating rubber of the belt and finally causes so-called xe2x80x9cheat separationxe2x80x9d between the cords and the coating rubber.
Also, when the tire is under load, the tread and the belt in the ground-contacting area of the tread are compressed radially inwards, and therefore the rubber coating reinforcing elements of the belt moves toward the belt edge portions. The quantity of displacement of the coating rubber becomes a maximum at the belt edge portions, especially at the edge portions of the circumferential belt layer, because the displacement of the coating rubber in the width direction is restricted. Therefore strain concentrates at the edge portions of the circumferential belt layer to cause separation.
It is, therefore, an object of the present invention to provide a tire which overcomes the disadvantage caused by addition of a circumferential belt layer, especially belt separation, while strengthening circumferential rigidity by the addition of the circumferential belt layer.
The present invention in one aspect provides a pneumatic tire comprising a belt and a tread disposed radially outside the crown portion of a carcass toroidally extending between a pair of beads, said belt being a laminate of a circumferential belt layer and an inclined belt layer, said circumferential belt layer comprising a plurality of reinforcing elements arranged substantially in parallel with the equatorial plane of the tire and coated with rubber, and said inclined belt layer comprising a plurality of reinforcing elements inclined with respect to the equatorial plane of the tire and coated with rubber, wherein at least as regards the coating rubber of said circumferential belt layer and the coating rubber of said inclined belt layer adjoining each other, the coating rubber of the circumferential belt layer has a lower modulus of elasticity than that of the coating rubber of the inclined belt layer.
It is advantageous for preventing belt edge separation that in the rubber layer between the reinforcing elements in the circumferential belt layer and those in the inclined belt layer adjoining each other in the vicinity of both edge portions of the belt, the thickness of the coating rubber of the circumferential belt layer accounting for the rubber layer is larger than the thickness of the coating rubber of the inclined belt layer.
The invention in another aspect provides a pneumatic tire comprising a belt and a tread disposed radially outside the crown portion of a carcass toroidally extending between a pair of beads, said belt being a laminate of a circumferential belt layer and an inclined belt layer, said circumferential belt layer comprising a plurality of reinforcing elements arranged substantially in parallel with the equatorial plane of the tire and coated with rubber, and said inclined belt layer comprising a plurality of reinforcing elements inclined with respect to the equatorial plane of the tire and coated with rubber, wherein a control rubber layer having a higher modulus of elasticity than that of the coating rubber of the circumferential belt layer is disposed outside of the widthwise edge of said circumferential belt layer.
As to the above circumferential belt layer, there can be applied not only the usual structure in which substantially straight cords or monofilaments as the reinforcing elements are arranged in the circumferential direction, but also a structure in which wavy cords or monofilaments are used and a structure in which a ribbon comprising several cords or monofilaments coated with rubber is spirally wound.